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## Jellybean Probability

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Hillel Just gettin' started!
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Tue May 10, 2011 9:46 am
Simply, 2 options out of 6 is 2/6 = 1/3
Simply, 3 options out of 6 is 3/6 = 1/2
Simply, 4 options out of 6 is 4/6 = 2/3

Since every box has the same probability to be chosen

it's an average (1/3 + 1/2 + 2/3) / 3 = 1/2

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rakeshvitta Just gettin' started!
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Thu Jul 21, 2011 9:06 pm
I did this in the below way:

For the first box, the probability of choosing the 2 flavors out of 6 is 2/6 and for the grape to be one of them it should be 1/2.- so grape to be in the first box- (2/6)(1/2)= 1/6
Similarly for the other boxes the probably for the grape to be present is 1/6.

Total probability = (1/6)+(1/6)+(1/6) = 1/2.

Is this a right way?

saketk GMAT Destroyer!
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Wed Sep 14, 2011 12:55 pm
total flavour = 6
3 boxes A,B,& C

A contains 2 flavours
2 can be selected out of 6 in 6C2 wauys = 15 ways
1 can be out of 5 and the other can be Grape = 5 ways

Probability :- 5 / 15 = 1/3 ways

B contains 3 falvours
Total - 6C3= 20 ways
1 is Grape, and 2 out of 5 can be selected in 5C2= 10
Probability = 12/20 = 1/2 ways

C contains 4 flavours

total 6C4 = 15 ways
1 is grape, other 3 can be selected out of 5 in 5C3 = 10 ways
Probability: - 10/15 = 2/3 ways

Also, we can have only one type of box at a time s0 -- any one type of box can be selected out of 3 in = 1/3 ways

ANSWER = 1/3 [(2/3)+(1/3)+(1/2)] = 1/2

Option D

immaculatesahai Rising GMAT Star
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Sat Dec 17, 2011 12:31 am
Ans is 1/2 as explained by Mitch earlier. I followed the same approach. Good question.

ankush123251 Rising GMAT Star
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Mon Dec 19, 2011 2:42 am
Good explanation.

Sharma_Gaurav Really wants to Beat The GMAT!
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Mon Jan 09, 2012 3:22 pm
good question. One important point to remember is that out of three boxes we are inly selecting on in the end. I got 45/30 and was wondering whatto do,
But then we have to multiply by 1/3 and hence P = 1/2 answer comes.

Option D is correct

shubhamkumar Rising GMAT Star
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Wed Mar 21, 2012 9:45 pm
pkw209 wrote:
A certain company sells jellybeans in the following six flavors only: banana, chocolate, grape, lemon, peach and strawberry. If the jellybeans are sorted randomly into boxes containing exactly 2, 3 or 4 different flavors only, what is the probability that any given box contains grape jellybeans?

a) 1/6

b) 1/3

c) 2/5

d) 1/2

e) 3/4
If the question is tweaked such that instead of the question asking
"what is the probability that any given box contains grape jellybeans?"
"what is the probability that atleast one of the boxes contains grape jellybeans?"
Wondering what the approach will here be

richaverma Just gettin' started!
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Thu Oct 18, 2012 1:17 pm
[quote="Testluv"]Let's consider the probability of not having grape, and then subtract from 1.

For the boxes with 2 flavors:

Prob = #des/#total

5C2/6C2 = 2/3
(the denominator is all the ways we can pull out any 2 flavors from the 6; the numerator is all the ways we can pull out 2 non-grape flavors from the 5 availabe)

For the boxes with 3 flavors: 5C3/6C3 = 1/2

For the boxes with 4 flavors: 5C4/6C4 = 1/3

Because the jellybeans are randomly assigned to each of the 3 kinds of boxes, the probability that you have any particular one kind of box is 1/3.

Thus, the probability of NOT having grape is: (1/3)*(2/3) + (1/3)*(1/2) + (1/3)*(1/3) = 1/2. Thus, the probability of selecting grape is also 1/2.

Choose D.

__________

The calculations are greatly sped up if you know that 5C4 = 5C1 = 5 (nC1 = n). Or that 5C3 = 5C2. Or that 6C4 = 6C2. Let's take 5C3 as an example. Every time we select a trio from 5, we are also "setting aside" a pair. Thus, 5C3 = 5C2. Similarly, 50C30 = 50C20, or 100C75 = 100C25, etc.

__________

Rather than using combinatorics, we could have also considered order. The probability of NOT having grape in the boxes with 2 flavors is (5/6)*(4/5) = 2/3, and so on. (Here, you can solve either by ignoring order--my first approach--or by considering order--my second approach. It doesn't matter, so long as you are consistent.)[/quote]
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Using combinatorial here suggests that a box cannot have multiple jellybeans of a single flavor. If a box with 2 jellybeans has both jellybeans of grape flavor that would meet the criteria too...correct?

So shouldn't the total number of ways we can have 2 jellybeans will actually be 6*6 and not 6C2?

Am I missing something here? I am getting 1/6 as my answer (by allowing repetitions) and I am not entirely convinced that using combinations is the right approach since this problem does involve repetitions. Please explain your technique further.

richaverma Just gettin' started!
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Thu Oct 18, 2012 1:20 pm
Using combinatorial here suggests that a box cannot have multiple jellybeans of a single flavor. If a box with 2 jellybeans has both jellybeans of grape flavor that would meet the criteria too...correct?

So shouldn't the total number of ways we can have 2 jellybeans will actually be 6*6 and not 6C2?

Am I missing something here? I am getting 1/6 as my answer (by allowing repetitions) and I am not entirely convinced that using combinations is the right approach since this problem does involve repetitions. Please explain your technique further.

mparakala Rising GMAT Star
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Thu Dec 20, 2012 2:29 pm
there are a total of 6 flavors
1) we need 2 out of 6 = 2/6
and probability of a grape jelly bean present in those 2 flavors = 1/2
2/6 * 1/2 = 1/6

2) we need 3 flavors out of 6 = 3/6
and one grape to be present in those 3 selected flavors = 1/3
3/6 * 1/3 = 1/6

3) we need 4 flavors = 4/6
and one grape to be present in those 4 flavors = 1/4
4/6*1/4 = 1/6

the grape can be present in (1) or (2) or (3)
so, 1/6 + 1/6 + 1/6 = 3/6 = 1/2

Ans D

Note: remember AND means MULTIPLICATION "*"

rajeshsinghgmat Really wants to Beat The GMAT!
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Sat Feb 09, 2013 6:19 pm

ConGMAT Just gettin' started!
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Sat Mar 02, 2013 7:34 pm
I am confused,
I am trying to know the probability of a grape jelly bean present in each box.
Can anyone help me? I dont know wich way is de correct one:

1) Probability of a grape gb present in BOX1: 2/6

2) Probability of a grape gb present in BOX1 : 1/6 + 1/5

Thanks!

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